The present invention relates to a continuous process for the direct production of polynitrated aromatic compounds by reacting an aromatic compound with a solution containing nitronium ions.
The production of nitrated aromatics has been the subject of numerous publications and patents. It has been known since 1846 that aromatic compounds can be converted to the corresponding substituted aromatic compounds with a mixture of sulfuric and nitric acid (i.e., the so-called mixed acid or nitrating acid). Musspratt & Hofmann, Liebigs Ann. Chem., Volume 57, page 201 (1846).
Nitrobenzene, dinitrobenzene, nitrochlorobenzene, nitrotoluene and dinitrotoluene have for many decades been commercially produced isothermally in stirred-tank reactors or in tubular reactors with a mixed acid composed of sulfuric acid and nitric acid. After a phase separation between the organic and aqueous phase, the nitrated aromatic product is recovered and the sulfuric acid is concentrated by evaporation of water at elevated temperature. Until now, polynitrations (for example dinitrations) have been carried out industrially by means of a two-step isothermal nitration. See, e.g., Kirk-Othmer, Encyclopedia of Chemical Technolgy, 3rd Edition, Volume 15 (1981) and Ullmann, Encyclopedia of Industrial Chemistry, Volume A17, pages 411-455 (VCH Weinheim (1991)).
The continuous (mono)nitration of aromatics has been described in detail in the prior art. See, e.g., Groggins, Unit Processes in Organic Chemistry (McGraw-Hill, New York (1958)). Mixed acids or separate streams of sulfuric acid and nitric acid together with, for example, benzene are fed into a stirred nitrator. The two-phase reaction mixture is stirred continuously and intensively cooled in order to conduct the reaction as isothermally as possible. From the nitrator this mixture is passed to another stirred-tank reactor connected in series, or is fed directly into a separator where the phases are separated. There the organic product phase is separated from the aqueous sulfuric acid phase and worked up. The sulfuric acid diluted by the water in the nitric acid and by the water of reaction has to be returned to the reactor as concentrated acid, with considerable expenditure of energy. In the polynitration of an aromatic compound, this energy-consuming concentration process is carried out in each individual, tandem-arranged, isothermal mononitration. In the dinitration of toluene, for example, the sulfuric acid may optionally be fed from the dinitration stage to the mononitration stage, so that only one concentration of sulfuric acid is necessary.
The physical and chemical data for these industrial nitrating conditions and the model concepts for nitration using nitronium ion solutions are discussed in the literature. See, for example, Hansen et al, Chem. Eng. Sci., Volume 32, page 775 (1977); Albright et al, ACS Symposium Series 22, page 201 (American Chemical Society (1976); Albright et al, J. App. Chem. Biotechnol., Volume 26, page 522 (1976); and Urbanski, Chemistry and Technology of Explosives, Volume 1 (MacMillan, New York (1964)).
The nitration is carried out in a two-phase reaction (organic aromatic phase and aqueous nitrating acid phase) essentially in the aqueous phase, so that the solubility of the aromatics in the aqueous phase, the rate of mass transfer from phase to phase and the intrinsic rate of reaction together influence the conversion rate observed as a whole. There thus exists a complex reaction system the rate of which is controlled kinetically or by the mass transfer, depending on how the reaction is conducted.
A model for the aromatic mononitration was formulated by Albright et al. and described in ACS Symposium Series 22, page 201 (American Chemical Society (1976)). This model was as follows:
a) unnitrated aromatic compound diffuses out of the organic phase along the organic/aqueous phase interface; PA0 b) unnitrated aromatic compound dissolves and diffuses from the organic/aqueous phase interface into the aqueous phase; PA0 c) nitric acid diffuses out of the interior of the aqueous phase in the direction of the phase interface; PA0 d) while the aromatic compound permeates into the aqueous phase and the HNO.sub.3 diffuses counter to it out of the interior of the aqueous phase, the aromatic compound reacts with HNO.sub.3 to form a nitroaromatic compound and water; PA0 e) the nitroaromatic compound formed diffuses back through the aqueous phase along the phase interface; PA0 f)the nitroaromatic compound formed dissolves along the phase interface and diffuses from the phase interface into the interior of the organic phase; and PA0 g) water formed diffuses from the site of formation into the interior of the aqueous phase.
Since the conventional isothermal mononitration processes are not the most energy efficient because the heat of reaction is first dissipated by cooling and the subsequent concentration of the acid requires a high energy input, early consideration was given to conducting the process adiabatically.
In U.S. Pat. No. 2,256,999 (1941) an adiabatic mononitration of several aromatic compounds such as benzene, is proposed as a new nitration process. Essential features of this prior art process are that one or more stirred-tank reactors are charged with a stoichiometric excess of the compound to be nitrated and consequently the nitric acid in the reactor is completely consumed. The sulfuric acid is then separated, concentrated using the heat of reaction and recycled to the reactor. The proportion of sulfuric acid used in the mixed acid is between 68 and 76 wt. %.
In U.S. Pat. No. 4,021,498 (1977), a process for adiabatic mononitration with an excess of nitric acid at mixing temperatures of between 40.degree. and 80.degree. C., sulfuric acid contents of from 60 to 70 wt. % and a maximum temperature below 145.degree. C. is described.
In U.S. Pat. No. 4,091,042 (1978), the conditions disclosed for operating the reactor are limited specifically to the mononitration of benzene. The operation is carried out using an excess of benzene of about 10 mol % as compared with contents of HNO.sub.3 and H.sub.2 SO.sub.4 of from 58.5 to 66.5 wt. %.
Adiabatic, continuous processes for the production of mononitroaromatic compounds have been described in U.S. Pat. Nos. 2,256,999 (1941); 4,021,498 (1977); and 4,091,042 (1978).
Alternatively, according to U.S. Pat. No. 3,928,475 (1975) and U.S. Pat. No. 3,981,935 (1976), the heat of nitration may be used by feeding benzene vapor and nitric acid to a stirred-tank reactor containing sulfuric acid and removing the vaporizing products, water and nitrobenzene together with benzene in vapor form from the reactor. In each of the disclosed processes, the heat of reaction is used to remove water. At least some of the heat reaction is wasted, however, in vaporizing large quantities of the educt and recycling it in condensed form.
EP-A 0,373,966 (1988; corresponds to U.S. Pat. No. 4,973,770) describes a process for the mononitration of organic substances using mixed acid which employs a droplet-producing liquid jet of organic material for mixing the two phases. The use of a liquid jet for liquid/liquid mixing during the mononitration is known. (See, e.g., U.S. Pat. No. 3,160,669 (1964).) The process disclosed in EP-A 0,373,966 is carried out using a deficit of HNO.sub.3, so that the aromatic compound is not completely nitrated in the reactor. In the one Example given, a conversion of only 55.3% for HNO.sub.3 and 52.5% for benzene is obtained in a reactor into which benzene is injected through a nozzle.
EP-A 0,436,443 (1990; corresponds to U.S. Pat. No. 5,313,009) discloses a continuous, adiabatic nitration process in which a mixed acid containing at least 55 mol % of H.sub.2 SO.sub.4 with 0% of HNO.sub.3 and at least 82 mol % of H.sub.2 SO.sub.4 with 18% of HNO.sub.3 is used. A nitration process in which an excess of aromatic compound (as compared with HNO.sub.3) is used is also claimed. In the description of the invention, it is emphasized that dinitration is to be avoided.
All of the above-described disclosures directed to adiabatic nitration processes focus on benzene in the actual Examples given and describe possible ways of conducting a process for a mononitration. More extensive nitrations are considerably more difficult to control because far more drastic reaction conditions and conditions for concentrating the sulfuric acid are necessary than is the case in a mononitration of, for example, benzene. See, e.g., Urbanski, Chemistry and Technology of Explosives, Volume 1 (MacMillan, New York (1964)). The second and especially the third nitro group are harder to introduce into an aromatic ring than is the first. So until now, higher temperatures and greater acid concentrations have been used for the isothermal production of diaromatics than for monoaromatics. See, e.g, Ullmann, Encyclopedia of Industrial Chemistry, Volume A17, pages 411-455 (VCH Weinheim (1991)).
In U.S. Pat. No. 5,001,272 (1989), a process for the production of a dinitrated aromatic compound is disclosed for the first time. Toluene is successfully converted to dinitrotoluene by means of highly concentrated aqueous nitric acid without other additives. High molar excesses of HNO.sub.3, and moderate temperatures of between 40.degree. and 70.degree. C. are necessary for this disclosed process.
U.S. Pat. Nos. 4,918,250 (1989) and 5,057,632 (1990) and WO-A 92/06937 (1990) disclose a two-step process for the nitration of toluene via the separate nitrotoluene intermediate step to form the end product dinitrotoluene. These disclosed processes are carried out using a high molar excess of highly-concentrated nitric acid for the dinitration.
U.S. Pat. Nos. 5,099,078 (1990); 5,099,080 (1991) and 5,245,092 (1992) each disclose a process for the dinitration of toluene using highly concentrated nitric acid in a single apparatus. In U.S. Pat. No. 5,099,080 (1991) a high molar excess of HNO.sub.3 is used (HNO.sub.3 :toluene equals from 12:1 to 9:1) and nitration is carried out at temperatures of from 0.degree. to 90.degree. C. In the process described in U.S. Pat. No. 5,245,092 (1992), the molar excess of HNO.sub.3 is even higher.
A continuous adiabatic process for dinitration with a mixed acid in which the heat of reaction is used to evaporate water in the nitric acid and in the product was disclosed for the first time in DE-A 4,238,390 (1993). In this disclosed nitration process, a nitric acid/sulfuric acid mixture which contains a proportion by weight of H.sub.2 SO.sub.4 of from 60 to 90%, a proportion by weight of HNO.sub.3 of from 5 to 20% and a molar ratio of HNO.sub.3 /toluene of at least 2.0 is used. No detailed information with respect to the reactor used is, however, given. It is stated in the Examples that toluene is successfully nitrated to dinitrotoluene in a thin tubular reactor having an internal diameter of 0.6 mm or 0.99 mm and a length of 20 m. A stoichiometric excess of HNO.sub.3 relative to toluene (molar ratio 2.15:1.0) is added and the yield is more than 99% dinitrotoluene (DNT). Mononitrotoluenes and trinitrotoluene are reported in very small quantities (&lt;1%). In the Examples, the reactor is operated at flow rates of from 1 to 3 l/h with very high pressure losses. Such pressure losses are technically difficult to control. The scale-up of a reactor of this type having laminar flow would be possible only by duplicating the single thin tubular reactors disclosed therein. This would, however, be very expensive. The advantage of the adiabatic mode of operation is reduced because of high heat losses due to the reactor construction which heat losses can be limited only by expensive insulation measures.
In heterogeneous reaction systems such as the systems in which aromatic compounds are nitrated using a mixed acid, inhibition of the rate of reaction frequently occurs due to mass transfer from one phase into the other. It is taught in Chem.-Ing.-Techn., Volume 56, pages 552-553 (1984) that two immiscible liquids can be finely distributed in each other as droplets by means of a pressure nozzle. The great increase in the interface between the two liquids makes it possible for chemical reactions between reactants in different phases to proceed more rapidly. The more finely dispersed the phases are distributed, the greater the increase in rate of reaction. High energy input into a two-phase system (e.g., through a nozzle) disperses a liquid jet into small drops immediately after leaving the nozzle orifice. Experimental results show that the sizes of the droplets in the disperse phase can be calculated from the energy input of any mixing device and from the data on the properties of the two liquids. Suitable mixing devices for dispersion are known. Examples of suitable mixing devices include jet mixers, static mixers and dynamic mixers. The most advantageous device will vary depending on the dispersion requirements. See, e.g., Ullmann, Encyclopedia of Industrial Chemistry, Volume B4, pages 561-586 (VCH Weinheim (1992)) and Koglin et al, Chem.-Ing.-Techn., Volume 53, pages 641-647 (1981).
Reduction of undesirable by-products in complex reaction systems comprising several parallel or successive reactions by micromixing the educts at a rate which is faster than the rate at which the educts react with one another has been described in the literature. See, for example, Ullmann ,Encyclopedia of Industrial Chemistry, Volume B2, Chapter 24 (VCH Weinheim (1992)); Bourne & Maire, Chem. Eng. Process, Volume 30, page 23 (1991); and Brodkey, Chem. Eng. Commun., Volume 8, page 1 (1981). If the chemical reaction rates and the rates of mixing the educts are of the same order of magnitude, then there is a complex interaction between the kinetics of the reactions and the local mixing behavior (the latter determined by the turbulence) in the reactor and around the mixing device. If the reaction rates are significantly faster than the mixing rates, the yields are clearly influenced by the mixing, that is, by the local velocity and concentration of the reactants and therefore by the construction of the reactor and the turbulence structure. See, e.g., Brodkey (ed.), Turbulence in Mixing Operations--Theory and Application to Mixing and Reaction (Academic Press, New York, (1975)).
Suitable devices for the rapid mixing of two liquid streams are described in many literature references and patents. See, e.g., Ullmann, Encyclopedia of Industrial Chemistry, Volume B4, pages 561-586 (VCH Weinheim (1992)). Many special devices have been developed for liquid/liquid mixing. Examples of such special devices are disclosed in U.S. Pat. Nos. 4,596,699; 4,647,212; and 4,361,407 and also EP-A 272,974. Special mixing devices for the adiabatic mononitration of benzene in tubular reactors have been disclosed in EP-A 0,373,966 and U.S. Pat. No. 4,994,242.
The device disclosed in U.S. Pat. No. 4,994,242 is a tubular reactor equipped with spherical redispersing baffles made of tantalum. Redispersing elements in the form of perforated plates, bubble plates, valve trays, dispersers, homogenizers, dynamic mixers, etc. are known to be useful for carrying out two-phase reactions. See, e.g., Schr oder et al, Chem.-Ing.-Techn, Volume 56, pages 552-553 (1981); Koglin et al, Chem.-Ing.-Techn, Volume 53, pages 641-647 (1981); and Koglin, Maschinemarkt, Volume 86, pages 346-350 (1980). U.S. Pat. No. 4,994,242 discloses a dispersing element characterized by high stability with minimum use of material (tantalum).